1. Field of the Invention
This invention relates to an improvement in an intake system of an internal combustion engine.
2. Description of the Prior Art
In the past, there have been proposed various types of four-cycle engines such as, for example, those disclosed in Japanese Unexamined Patent Publication Nos. 54(1979)-65207 and 50(1975)-107309, wherein an engine cylinder head is recessed at a location surrounding an intake port or an exhaust port and an ignition plug so as to provide a combustion chamber between the inner wall surface of the cylinder head and the top surface of a piston at its top dead-center, with a large squish area of a limited clearance formed adjacent the recess.
In the above-described engines, since the large squish area is provided, the mixture in the large squish area is pushed into the combustion chamber by the upward movement of the piston in the compression stroke, thereby creating a squish flow and a great disturbance of mixture. Hence, the combustion speed is enhanced and the combustion efficiency during the light-load operation is improved, thereby improving the fuel consumption.
Further, in the above-described engines, because of the compact combustion chamber, the flame-propagation distance is made short advantageously from the viewpoint of anti-knocking, whereby it is made possible to enlarge the compression ratio. By enlarging the compression ratio, the output power is enhanced and the fuel consumption is improved.
In the above-described engines, however, if a large squish area and a small combustion chamber are formed in order to sufficiently increase the combustion efficiency, the combustion noises are increased during the heavy-load operation demanding a large air intake.
On the other hand, the present invention have discovered that in the above-described type of engines the disturbance in the combustion chamber immediately before combustion is further enhanced if the air intake introduced in the intake stroke is guided in the direction along the periphery of the cylinder to form a strong swirl. However, if the swirl during the intake stroke is made strong enough for the light-load operation, the combustion noises during the heavy-load becomes too large.